The desire to “see” in complete darkness
or through obscurants such as
smoke or fog has driven the development
and adoption of thermal imaging
technology. Thermal imaging is the
translation of a scene’s heat signature —
the 8-μm to 14-μm or long-wavelength
infrared (LWIR) energy an object emits
— into a visible image or data that can
be interpreted by a computer.

Because the thermal energy of a scene
is largely independent of reflected light
and because it can travel through many
obscurants, thermal imaging is the technology
of choice for imaging in the dark
or other difficult environmental conditions. The demand of infrared vision is
there but has been hampered by the high
cost of conventional thermal imaging
cameras. A new passive optical component
— the thermal light valve (TLV) —
has entered the thermal imaging market
with the intent of reducing the cost of
thermal imagers.

Optical thermal imaging based on thermal
light valve technology does not rely
on the change of resistance, or electrical
effects, to measure temperature changes.
Instead, the thermal light valve imaging
technique relies on changes in optical
properties when exposed to temperature
changes. These optical-property changes
are detected using standard digital camera
electronics rather than electrically
reading the signal from the sensor itself.

The TLV is composed of narrow-band
optical filter pixels standing on thermally
isolating posts on a standard
MEMS (microelectromechanical system)
substrate (see Figure 2). Each pixel
acts as a passive wavelength converter.
Long-wavelength infrared (LWIR) radiation
from the scene is imaged onto and
absorbed by the TLV. This heats up thermal
pixels on the array in direct relation
to the thermal signature of the scene. The reflective wavelengths of the pixels
shift based upon the thermal energy incident
on each. A narrow-band near-infrared
(NIR) light source is used to
“probe” the temperature of the pixels
across the TLV. This NIR probe signal is
reflected off the TLV in varying
amounts, depending on the pixel temperature,
onto the CMOS imager. The
intensity of the light received by the
CMOS imager is therefore “modulated”
by the heat signature of the scene.

The TLV tunable optical filter is a
Fabry-Perot (FP) structure. It is constructed
from amorphous silicon (a-Si)
and silicon nitride (SiNx) thin films,
which have been used extensively for
many years in solar cells and flat-panel displays.
These materials are deposited using
plasma-enhanced chemical vapor deposition
(PECVD), which is capable of producing
uniform, dense materials in high-volume
manufacturing environments.
The optical filter’s minimum reflective
wavelength depends on the optical thickness
of the cavity — a product of physical
thickness and index of refraction.

The TLV method achieves tunability
by changing the index of refraction. The
materials are characterized by a high
thermo-optic coefficient, which is defined
as the change of index of refraction
per degree of temperature change.

Some important key differences compared
to other technologies are:

The sensing array is not an electronic
device. It is purely a passive layer of optical
thin films on glass. This greatly simplifies
manufacturing and packaging.

The sensing array is manufactured in a
standard MEMS foundry, thereby taking
advantage of foundry economies of
scale to dramatically reduce manufacturing
cost over that obtainable with
custom fabrication lines.

The readout circuit in the optical thermal
imaging system is not physically coupled
to the sensing array, nor is it a custom
design. It consists of off-the-shelf
parts, such as laser diodes and CMOS
sensors that can be sourced from high-volume
optical mouse and consumer
camera applications, and managed independently
from the sensor array. This
reduces cost, increases yield, and reduces
development cycle time.

This article was written by Daniel Ostrower,
Senior Director of Product Management
at RedShift Systems, Inc. For
more information, contact Mr. Ostrower at
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